Electronic devices sometimes experience overload conditions due to faults or shorts in one or more circuits. If an overload condition is not effectively eliminated very quickly, it can cause intense localized resistive heating to occur in one or more circuits which can damage or destroy the electronic device. To protect electronic devices from the dangers of overload conditions, standard fuses, which generally comprise a thin strip or ribbon of metal, have been used for many years. When an overload condition occurs in a circuit protected by a standard fuse, resistive heating generated by the excessive current flow through the thin strip or ribbon of metal causes it to melt and disjoin thereby breaking the circuit and protecting the device. In essence, the integrity of the standard fuse is sacrificed in order to protect the integrity of the electronic device.
Although standard fuses are highly effective in protecting electronic circuits, they are not practical for many applications, particularly miniaturized modern electronic devices. A standard fuse is a one time use device that must be replaced after an overload condition has been cleared because the thin strip or ribbon of metal cannot be rejoined after it has melted through. In some applications, the expense incurred to locate and replace a standard fuse can exceed the value of the electronic device that it was intended to protect.
It was discovered many years ago that materials exhibiting a positive temperature coefficient of resistance (PTC) could be utilized to form electronic circuit protection devices that overcome the one-time-use limitation of standard fuses. PTC circuit protection devices are generally formed from materials comprised of conductive particles dispersed in an organic polymer matrix. At normal current flows and temperatures, the conductive particles form chains or paths in the polymer matrix to create a polymer composite with high electrical conductivity. However, when the material is exposed to excessive current flows, resistive heating generated by the excessive current flow through the conductive particle chains causes the polymer to self-heat above its glass transition temperature (T.sub.g). The increase in temperature beyond the glass transition temperature of the polymer causes the polymer to expand and thereby separate some of the chains of conductive particles dispersed therein. This results in an increase in the electrical resistance of the material and substantially reduces the amount of current which flows through the PTC device into the protected circuit. Once the overload condition is cleared, the polymer self-cools and contracts where the conductive particles again form conductive paths to resume low resistance electrical current conduction.
Although known PTC devices effectively overcome the one-time-use limitation of standard fuses, they present new problems. The switching effect of most PTC devices is based upon the glass transition temperature of the conductive particle filled polymer. Thus, the switching effect is dependent in large part upon the degree of crystallinity of the polymer, which is generally a function of its heat history. A polymer that has been subjected to heating above its glass transition temperature will generally exhibit a higher degree of crystallinity upon cooling as compared to a similar polymer which has not been exposed to such heating. Thus, one of the problems with prior art PTC circuit protection devices is that each time the material goes through a heat cycle as a result of a current overload condition, the degree of crystallinity of the polymer is incrementally increased when it returns to its low resistance state thus resulting in a change in the switching temperature of the device for the next switching cycle and sometimes the initial resistance.
Another limitation of PTC circuit protection devices is that the material does not completely break the flow of current to the device or circuit being protected. At or above the switching temperature, the increased resistance of the PTC material substantially lowers the current flowing through the device, but some electrical current continues to pass through or leak into the electronic device or circuit being protected. Thus, the switching effect of known PTC devices is only a few decades (i.e., a few times 10.sup.X ohms). In certain applications, the continued flow of electrical current into the device during an overload condition can be detrimental to the integrity of the device.
Yet another limitation of PTC circuit protection devices arises from the manner in which they must be installed in electronic devices by manufacturers. The customary method of constructing a PTC device generally comprises extruding a polymer having an electrically conductive material dispersed therein into a film or sheet. Metallic electrode plates, usually metal foils or screens, are then applied to opposite sides or ends of the extruded conductive particle filled polymer film and bonded to the extruded polymer by heat and pressure. The metal plated polymer film is then cut into strips and the metal foil electrode plates are welded or soldered into a metal case. PTC devices of this type are then shipped to manufacturers who mount them in electronic devices using soldering techniques. This introduces space limitations in circuit design. Moreover, soldering or welding of this nature must be very precise in order to properly position the PTC device in the circuit without damaging it or the circuit in the electronic device. Another inherent problem with soldering or welding prior art PTC devices into electronic devices is that it causes the whole of the PTC device to be heated during installation. This can cause a portion of the PTC polymer to pyrolyze resulting in the evolution of gas which can weaken the bond between the conductive particle filled polymer composition and metal foil electrode plates. This weakening of the bond between the electrode plates and the polymer, coupled with the additional heating of the polymer itself, which can result in an increase in the crystallinity of the polymer, can increase the resistance of the PTC device beyond that which it was originally intended to provide.
A circuit protection device or fuse is needed which can overcome the limitations of standard fuses and the limitations of electrical circuit protection devices formed from known PTC materials. Ideally, such a circuit protection device would be formed from a composition that could be screen-printed directly onto a printed circuit in a variety of shapes or patterns to form resettable fuses that do not exhibit changes or variations in their predetermined switching temperatures over time.